When properly designed and implemented, automobile air bag restraint devices can dramatically reduce the injury and loss of life caused by automobile collisions. A properly deployed air bag may cushion a driver or passenger, thereby reducing the risk of injury. Proper air bag deployment includes rapidly inflating the air bag to a volume sufficient to cushion the occupant. Air bag inflator elements and their design and operation are described in the commonly owned and copending application for Method and System for Evaluating Gas Generants and Gas Generators, application Ser. No. 08/179,727, filed Jan. 10, 1994, which is incorporated herein by reference. The terms "gas bag" and "air bag" are used interchangeably herein.
Gas bags are typically inflated by combustion of a chemical gas generant. As the gas generant combusts, it produces gases and other combustion products which rapidly increase both the temperature and pressure within the air bag. Automobile manufacturers and others have set forth detailed specifications for gas generants. Preparing gas generating compositions that meet these important design criteria is an extremely difficult task. These specifications require that the gas generating composition produce gas at a required rate. The specifications also place strict limits on the generation of toxic or harmful gases or solids. Examples of restricted gases include carbon monoxide, carbon dioxide, NOx, SOx, and hydrogen sulfide.
The automobile manufacturers have also specified that the gas be generated at a sufficiently and reasonably low temperature so that the occupants of the car are not burned upon impacting an inflated air bag. If the gas produced is overly hot, there is a possibility that the occupant of the motor vehicle may be burned upon impacting a just deployed air bag. Accordingly, it is necessary that the combination of the gas generant and the construction of the air bag isolates automobile occupants from excessive heat. All of this is required while the gas generant maintains an adequate burn rate. In the industry, burn rates in excess of 0.5 inch per second (ips) at 1,000 psi, and preferably in the range of from about 1.0 ips to about 1.2 ips at 1,000 psi are generally desired.
Another related but important design criteria is that the gas generant composition produce a limited quantity of particulate materials. Particulate materials can interfere with the operation of the air bag restraint system, present an inhalation hazard, and irritate the skin and eyes. The spreading of potentially harmful dust in the vicinity of the spent air bag can cause lung, mucous membrane, and eye irritation to vehicle occupants and rescuers.
At present, sodium azide is the most widely used and accepted gas generating material. Sodium azide nominally meets industry specifications and guidelines. Nevertheless, sodium azide presents a number of persistent problems. Sodium azide is relatively toxic as a starting material, since its toxicity level as measured by oral rat LD.sub.50 is in the range of 45 mg/kg. Workers who regularly handle sodium azide have experienced various health problems such as severe headaches, shortness of breath, convulsions, and other symptoms.
In addition, sodium azide combustion products can also be toxic since molybdenum disulfide and sulfur are presently the preferred oxidizers for use with sodium azide. The reaction of these materials produces toxic hydrogen sulfide gas, corrosive sodium oxide, sodium sulfide, and sodium hydroxide powder. Rescue workers and automobile occupants have complained about both the hydrogen sulfide gas and the corrosive powder produced by the operation of sodium azide-based gas generants.
One known approach to reducing the concentrations of toxic compounds and particulates within the inflated gas bag is to entrain air from inside the vehicle and mix it with the gas generant combustion products during gas bag inflation. The entrained air dilutes the output of the gas bag generator, and may reduce the amount of gas generant needed to inflate a bag to a specified volume within a specified time. Heaters may also be provided in the inflator to increase pressure and reduce the gas generant mass flow rate needed for acceptable inflation. Another known approach to reducing toxicity and particulate concentrations is to mix the gas generator output with a diluting gas stored in a pressurized chamber within the gas bag inflator, such as in a so-called hybrid inflator. In these cases, however, the undesirable concentrations are merely reduced, and are not eliminated.
The bag is provided with a venting means through which gases vent into the interior of the car when the pressure inside the bag exceeds ambient pressure. The venting means may include open vents up to about two inches in diameter through the membrane of the gas bag. The venting means may also include use of a porous material as part of the bag membrane. The porous material is typically installed on the side of the bag opposite the driver or passenger, so the bag does not vent directly in their faces but nonetheless vents into the interior of the vehicle.
Venting begins on inflation, typically as soon as the pressure within the bag exceeds ambient pressure by as little as one pound per square inch. Forced venting, caused by the impact of an occupant against the bag membrane, uses the occupant's kinetic energy to deflate the bag, thereby reducing the energy that carries the occupant into an interior surface of the car. The impact of the occupant against the bag therefore forces toxic PG,6 compounds and particulates within the gas bag to be vented into the interior of the car.
Thus, it would be an advancement in the art to provide a system and method for inflating and deflating a gas bag which do not substantially expose a vehicle's occupants to gas generant compositions or to the combustion products of such compositions.
Such a system and method are disclosed and claimed herein.